I understand the point you are making; all kinds of scenarios can be imagined.Consider this ,
a farmer keeps getting his truck drained , so he does nothing but keep filling it up , a 13 year old boy knows his dad gets his diesel from the farm , and its easy no risk , so he goes and nicks some himself and puts it in his dads van takes it out for a joyride , but coz his dad doesnt maintain the van as hes got no care for anyone else the brakes fail and the lad goes down the road too fast cant brake for the bend and goes over a cliff the farmer feels guilty for not putting a 24 hour guard on the fuel but they all lived happily ever after !
However you are missing the point of intent.
In the scenario you create the farmer did not intend for any of it to happen.
In the actual case you first explained the farmer did intend to cause vehicle breakdown. With today's motorways without hard shoulders it is not too far fetched to imagine serious consequences of the farmer's actions to completely innocent people.
Also don't forget the farmer is a thief too. He is stealing from the Government with his red diesel usage.